update fifo per PanRe review

src/common/tusb_fifo.h

@@ -54,7 +54,7 @@ extern "C" {
 
 /* Write/Read index is always in the range of:
  *      0 .. 2*depth-1
- * The extra window allow us to determine the fifo state of empty or full with only 2 indexes
+ * The extra window allow us to determine the fifo state of empty or full with only 2 indices
  * Following are examples with depth = 3
  *
  * - empty: W = R
@@ -86,7 +86,8 @@ extern "C" {
  *      -------------------------
  *      | R | 1 | 2 | 3 | W | 5 |
  *
- *  - Double Overflowed: write(3), write(1), write(2)
+ *  - Double Overflowed i.e index is out of allowed range [0,2*depth) e.g:
+ *      write(3), write(1), write(2)
  *    Continue to write after overflowed to 2nd overflowed.
  *    We must prevent 2nd overflowed since it will cause incorrect computed of count, in above example
  *    if not handled the fifo will be empty instead of continue-to-be full. Since we must not modify
@@ -94,7 +95,7 @@ extern "C" {
  *    after data is written it is a full fifo i.e W = depth - R
  *
  *      re-position W = 1 before write(2)
- *      Note: we should also move data from mem[4] to read index as well, but deliberately skipped here
+ *      Note: we should also move data from mem[3] to read index as well, but deliberately skipped here
  *      since it is an expensive operation !!!
  *                  |
  *      -------------------------